This invention relates to automatic control systems for nuclear reactors and more particularly to inherent shutdown systems for nuclear reactor control rods.
All nuclear reactors incorporate an emergency shutdown system which comprises a device for reducing the reactivity of the nuclear reactor core. The reduction in reactivity of the nuclear reactor core is generally accomplished by way of introducing a form of neutron absorbing material into the nuclear reactor core. The systems are intended to be simple and reliable systems for quickly introducing neutron absorbing material into the core of the nuclear reactor for effecting a shutdown of the reactor. The basic method known in the prior art for controlling a nuclear reactor is the system comprising control rods. In this system, the control rods are raised and lowered by mechanical mechanisms into and out of the nuclear reactor core for controlling the amount of reactivity in the core. In an emergency situation, the control rods are released such that they fall into the reactor core area thus reducing the reactivity of the core and shutting down the reactor. While this system has proven to be extremely reliable, additional back-up systems have been proposed as a redundant emergency reactor shutdown system. In many of the additional back-up systems proposed, the system is designed such that it will be a self-actuating or an inherent system.
As an alternative method of reactor control, the use of neutron absorbing balls has been suggested. In one such control system, the apparatus comprises a restraint mechanism for releasably restraining a multitude of discrete bodies of materials which are magnetic and possess neutron absorbing properties. The restraint means comprises at least one pair of magnetic pole pieces of opposite polarity, the apparatus being operated to release the discrete bodies by demagnetization of the pole pieces whereby the bodies are moved under the influence of gravity into the core of the reactor. In this system, should the magnetization of the pole pieces be lost for whatever reason, the neutron absorbing balls are dropped into the reactor core thereby reducing the reactivity of the nuclear reactor core.
Still another alternate method of reactor control comprises the use of a Curie point material which when heated to the Curie point loses substantially all of its magnetic susceptibility thereby causing an activation of the control system. In one such system, the neutron absorber material is restrained and supported by a magnet whose lines of force are linked by a yoke. The yoke is of a ferromagnetic material having a Curie point at the critical level and in contact with the reactor coolant. When the reactor coolant temperatures reaches the critical level, the yoke will be at its Curie point and thereby will lose its magnetic susceptibility releasing the neutron absorbing material into the core of the nuclear reactor. When cooled, the yoke regains its magnetic susceptibility thereby allowing the magnetic latch to be reused. One difficulty with the Curie point type systems is that the Curie material requires a relatively long time to heat to the Curie point due to the mass of the components. The slowness of response time of the Curie material may result in the inability of the reactor shutdown system to respond quickly.
Therefore, what is needed is a nuclear reactor shutdown system that is capable of quickly shutting down the nuclear reactor when the reactor coolant rises above a specific temperature.